A WWW server is an excellent target for hackers and for other miscreants who desire to have their exploits publicized. The server usually has a captive audience that downloads information (usually WWW pages) from the server. By modifying the information sent out by the server, such a miscreant publishes his exploits. For the owner of the server, the damage is disproportionate—the credibility of the server is severely reduced. In addition, erroneously published information may directly harm the server's owner, for example, by misrepresenting prices of services. The server owner would like to be able to stand behind what is “published” at the WWW site.
There appear to be two approaches in the art to avoiding interference with WWW services. A digital signature approach puts the onus on the receiver of a document (data) to verify that the document is what it purports to be. If the document does not match its attached signature, the receiver can assume that the document is bogus or corrupted. This solution, however, requires that the receiver be able to validate the document using the signature, typically requiring a copy of a public encryption key used by the signing protocol.
A security approach attempts to nullify the possibility of an outside break-in into the WWW server, for example using a firewall, so that it can be assumed that any information provided by the server is not adulterated by an outside hacker. Unfortunately, constructing a completely secure system is difficult, if not impossible, and miscreants are unusually creative in their efforts to “hack” into supposedly secure systems. An even greater problem is internal security. A disgruntled employee can bypass many security features by working at his computer terminal, inside the server physical location or by using a password which is known to him.
In addition, some secure systems disseminate information to a requester only after the requester's identity has been verified (usually using a password) and his permission to access the information confirmed.
Some types of firewall block requests for transmission of certain named files.
U.S. Pat. No. 4,672,572, the disclosure of which is incorporated herein by reference, describes various protection schemes for computer networks. One of the described schemes is a command filter which can monitor data transfers which pass through it and detect, block or modify sensitive information being transferred or sensitive commands from being carried out.
Recently, mail servers have been patched with software code that prevents the transmission of messages which appear to contain certain viruses.
The tripwire software and various virus detection software maintain a list of signatures of files. If one of the files is corrupted and does not match its signature, a system operator may be altered. In a virus detection system such a determination of mismatch may be made when a file is loaded into a computer memory for execution. In some systems, files are checked against their signatures periodically.